1. Field of Invention
The present invention relates to radioactive logging of subsurface well formations of interest.
2. Description of Prior Art
In petroleum exploration and production, it has been very helpful to determine permeable intervals or zones in what were otherwise unproductive formations. Those of particular interest were naturally fractured intervals in shale, carbonate or sandstone reservoirs. These naturally fractured zones were typically the primary productive intervals in the formations of interest. Where the formation was an impermeable rock a naturally fractured zone if located could be a source of high oil or gas yield. One type of exploration drilling where location of such a zone would be particularly beneficial, and in which there has arisen considerable interest, would be that of horizontal drilling.
Permeability might be in the form of natural fractures or in the form of sharp contrasts, as where a thin sandstone formation was present within a thick shale sequence. A naturally fractured zone or zones might be only a small percentage of an overall interval under investigation. Discontinuous gas sands have occurred as relatively thin (five to twenty feet) channel type deposits within a thick shale package. Open hole well logs have been run in attempts to locate these areas of interest. So far as is known, these types of naturally fractured zones have been difficult to detect or delineate with known open hole well logs.